1. Field of the Invention
The present invention relates generally to phase angle balancing circuits for multiphase electric power systems, and more particularly, to a phase angle balancing circuit for a variable speed constant frequency (VSCF) power system where the zero sequence voltage of the multiphase electric power signals from the cycloconverters is produced continuously and automatically to effect phase angle balancing between the multiphase electric power signals. The present invention is specifically directed to a phase angle balancing circuit for a VSCF multiphase electric power system used in an aircraft environment.
2. Description of the Prior Art
Phase angle balancing has been a problem which has existed in multiphase VSCF electric power systems from their inception. Phase angle balancing is required by the users of these generators, who require it for equipment which is phase error sensitive. The phase angle imbalance in such multiphase VSCF electric power system is caused by changing system loads and component value variations of the components in the multiphase electric power generation system itself.
In the area of variable speed constant frequency (VSCF) multiphase electric power generation systems, which are used extensively in aircraft applications, the problems which result from improper phase angle relationships between the multiphase electric power signals becomes acute due to the problems which result in the performace of the electronically sophisticated loads connected to the VSCF power system.
In conventional VSCF systems, feedback circuits from the output of each cycloconverter and its associated modulator circuit together with individual voltage amplitude and frequency reference signals allow the voltage amplitude levels of the multiphase electric power signals to be kept automatically and continuously within specified limits. However, when the loads connected to the VSCF system power signals become imbalanced, or when the component values of the components used in the VSCF system deviate from the preselected values, a phase angle imbalance between the multiphase electric power signals occurs. Such phase angle imbalances are uncorrectable in conventional VSCF systems.
While it has been impossible as a practical matter to prevent phase angle imbalance caused by varying loads in conventional VSCF power systems, many attempts for improving the phase angle balancing of the multiphase electric power signals when the loads are constant and balanced have been made by keeping the tolerances of the components in the conventional VSCF system within tight limits. Such tight tolerances of component parameters, however, result in substantial increases in fabrication, operation and maintenance costs, and, thus, should be eliminated, if possible. Representative of conventional VSCF power systems and subsystems which do not exhibit continuous and automatic phase angle balancing are the following references:
______________________________________ Re. 26,630 Peaslee Issued 7/15/69 3,152,297 Peaslee Issued 10/6/64 3,289,070 Caldwell Issued 11/29/66 et al 3,400,321 LaFuze Issued 9/3/68 3,419,785 LaFuze Issued 12/31/68 3,593,106 LaFuze Issued 7/13/71 3,609,509 LaFuze Issued 9/28/71 3,641,418 Plette Issued 2/8/72 3,745,471 LaFuze Issued 7/10/73 3,873,928 LaFuze Issued 3/25/75 3,902,073 LaFuze Issued 8/26/75 3,908,161 Messenger Issued 9/23/75 ______________________________________
An excellent generalized discussion of conventional VSCF power systems is found in Aircraft Electric Power Seminar--Technical Proceedings, General Electric Company, Aircraft Equipment Division, Binghamton, N.Y., May 10-11, 1977.
U.S. Pat. No. 4,044,296 to Dhyanchand et al, issued Aug. 23, 1977, discloses an electronic voltage regulator for three-phase generators. Dhyanchand et al is only of interest with respect to the present invention for its teaching of sensing the average of the three-phase voltages. The purpose, function, and structure of the Dhyanchand et al system, however, are entirely different from those of the present invention. Specifically, in Dhyanchand et al, the purpose of the system is to provide electronic circuits for regulating the voltage and for limiting the current output of the three-phase generators, whereas the purpose of the phase angle balancing circuit for a VSCF system of the present invention is to maintain continuously and automatically the phase relationship between the multiphase electric power signals. The function of the Dhyanchand et al system is to provide electronic voltage regulation by sensing the average of the phase voltages, the highest phase voltage and the highest phase current, and by comparing the error signal with a triangular wave to provide a signal to control an output amplifier circuit and ultimately the exciter field current of the generator. In comparison, the zero sequence voltage signal produced by the phase angle balancing circuit for a VSCF power system of the present invention is applied having a polarity to produce negative feedback to each of the modulators associated with the respective cycloconverters. Examination of the circuit diagrams in Dhyanchand et al and the present invention shows that they are entirely different.